struct S
{
    void* p;
}

void main()
{
    S s;
    auto a = cast(void*)s; //Error: e2ir: cannot cast s of type S to type void*
}

Is there are a good reason for this being disallowed?

On Friday, 9 January 2015 at 18:25:42 UTC, John Colvin wrote:
> struct S
> {
>     void* p;
> }
>
> void main()
> {
>     S s;
>     auto a = cast(void*)s; //Error: e2ir: cannot cast s of type S to type void*
> }
>
> Is there are a good reason for this being disallowed?

You'd expect `cast(void*)s == s.p`? That doesn't work for any type of p. You can do it with a slightly fancier (and more dangerous) cast: `*cast(void**)&s`.

On 01/09/2015 10:25 AM, John Colvin wrote:
> struct S
> {
>      void* p;
> }
>
> void main()
> {
>      S s;
>      auto a = cast(void*)s; //Error: e2ir: cannot cast s of type S to
> type void*
> }
>
> Is there are a good reason for this being disallowed?

I know two options:

a)

    alias p this;

b)

    auto opCast(T : void*)()
    {
        return p;
    }

Ali

On Friday, 9 January 2015 at 18:35:56 UTC, anonymous wrote:
> On Friday, 9 January 2015 at 18:25:42 UTC, John Colvin wrote:
>> struct S
>> {
>>    void* p;
>> }
>>
>> void main()
>> {
>>    S s;
>>    auto a = cast(void*)s; //Error: e2ir: cannot cast s of type S to type void*
>> }
>>
>> Is there are a good reason for this being disallowed?
>
> You'd expect `cast(void*)s == s.p`? That doesn't work for any type of p.

I was expecting it to work regardless of the type of p. I have an 8 byte (on x86_64) struct which I want to reinterpret as a void*

> You can do it with a slightly fancier (and more dangerous) cast: `*cast(void**)&s`.

Yuk. Better than nothing though. Thanks :)

On 1/9/15 1:50 PM, John Colvin wrote:
> On Friday, 9 January 2015 at 18:35:56 UTC, anonymous wrote:
>> On Friday, 9 January 2015 at 18:25:42 UTC, John Colvin wrote:
>>> struct S
>>> {
>>>    void* p;
>>> }
>>>
>>> void main()
>>> {
>>>    S s;
>>>    auto a = cast(void*)s; //Error: e2ir: cannot cast s of type S to
>>> type void*

This is actually a compiler bug! I will check to make sure it's not already filed, and file if it's not. However, I don't think the code should work, it just shouldn't print e2ir.

>>> }
>>>
>>> Is there are a good reason for this being disallowed?
>>
>> You'd expect `cast(void*)s == s.p`? That doesn't work for any type of p.
>
> I was expecting it to work regardless of the type of p. I have an 8 byte
> (on x86_64) struct which I want to reinterpret as a void*
>
>> You can do it with a slightly fancier (and more dangerous) cast:
>> `*cast(void**)&s`.
>
> Yuk. Better than nothing though. Thanks :)

This is what reinterpret_cast from C++ does.

-Steve

On Friday, 9 January 2015 at 19:03:04 UTC, Steven Schveighoffer wrote:
> On 1/9/15 1:50 PM, John Colvin wrote:
>> On Friday, 9 January 2015 at 18:35:56 UTC, anonymous wrote:
>>> On Friday, 9 January 2015 at 18:25:42 UTC, John Colvin wrote:
>>>> struct S
>>>> {
>>>>   void* p;
>>>> }
>>>>
>>>> void main()
>>>> {
>>>>   S s;
>>>>   auto a = cast(void*)s; //Error: e2ir: cannot cast s of type S to
>>>> type void*
>
> This is actually a compiler bug!

So it is! The same happens with e.g. casting void* to string. Annoyingly it passes __traits(compiles, ...)